1. FIELD of THE INVENTION
The present invention relates to a semiconductor device and more particularly to a semiconductor integrated circuit device in which a plurality of circuit elements are formed in a single substrate.
2. DESCRIPTION OF THE PRIOR ART
In general, a semiconductor integrated circuit device is fabricated and endowed with an excellent function, by forming a multiplicity of circuit elements such as resistors, diodes and transistors in a single substrate comprising silicon polycrystalline regions or silicon polycrystalline and monocrystalline regions and by electrically wiring the circuit elements. In such a case, the circuit elements must be isolated from one another in order to prevent mutual interference. A conventional way of isolating or insulating such circuit elements from one another is known as PN junction isolation method in which PN junctions are maintained in reversely biased conditions. In the PN junction isolation method, however, parasitic active and/or passive elements are formed between the circuit elements and the substrate. Therefore, this method has a drawback of lacking a desired efficiency at high frequencies, for a certain application, e.g. one to speech path elements in a communication apparatus treating high frequency signals. This method also has another drawback that if AC signals are treated PN junctions are reversely biased to invalidate the function of the PN junction of isolating the circuit elements.
There has been used a conventional dielectric isolation method to obtain a high efficiency and an excellent function, in which a dielectric film of a material such as SiO.sub.2 is used as an insulating medium. In this dielectric isolation method, however, the isolation structure is more elaborate in fabricating techniques than in the PN junction isolation method, so that the integration density cannot be so much increased to maintain the dimensional precision.